The fibroblast growth factor (FGF) family of proteins exert an array of biological effects both in vitro and in vivo. Two members of this family of polypeptides, acidic and basic FGF, have been the most extensively characterized. The effects of these growth factors on cultured cells include regulation of proliferation and differentiation, maintenance of the differentiated state, stimulation of cellular migration, and prolongation of survival of certain cell types. The functions of FGF in vivo are postulated to include embryonic induction, stimulation of angiogenesis, promotion of regenerative processes such as wound healing, and stimulation of tumor growth and metastases. Exogenously supplied FGFs inhibit skeletal myoblast differentiation in vitro. Cardiac and skeletal myocytes synthesize FGFs, and during skeletal myogenesis a coordinate decrease in the expression of acidic and basic FGF and of their cognate receptor occurs. It has recently been demonstrated that overexpression of an FGF delays the differentiation of skeletal myoblasts, while inhibition of FGF expression by antisense techniques triggers myogenesis. The effects of FGF on cardiac myogenesis are unknown. One goal of this proposal is to determine the mechanisms whereby acidic and basic FGF exert their effects on myogenic differentiation. Recent evidence suggests that the FGFs may exert their effects through either stimulation of surface receptors possessing tyrosine kinase activities, or through direct nuclear effects. The sequences of the FGF proteins responsible for cellular trafficking will be identified through site directed mutagenesis of the cDNAs encoding acidic and basic FGF. The effects of directed nuclear, cytoplasmic, and extracellular localization on skeletal myocyte proliferation and differentiation will be determined. A second goal is to determine whether the decreased FGF receptor expression which occurs during myogenesis serves a regulatory role in this developmental process. The influence of increased FGF responsiveness on myogenesis will be determined by two approaches. In the first, FGF receptor expression will be constitutively increased in myoblasts to determine if differentiation is inhibited. In the second approach, the expression of functional receptors will be inhibited through a dominant negative strategy employing receptor mutants lacking tyrosine kinase domains to determine if decreased FGF receptor responsiveness triggers myogenesis. The final goal of the proposal is to determine the influence of endogenously synthesized FGF on cardiac and skeletal muscle development in vivo. The model systems to be used are strains of transgenic mice which have recently been developed that constitutively overexpress acidic and basic FGF in cardiac and skeletal muscle. The analysis of these transgenic mice is anticipated to elucidate the role of these heparin binding growth factors in cardiac and skeletal muscle development.